1. Field of the Invention
The present invention relates to the heat treatment of a nickel-based superalloy known under the trade name of "718" and having a typical composition comprising, in percentages by weight, Cr=19, Fe=18, Nb=5, and mainly Ni as the remainder.
This alloy is used in particular in the manufacture of highly stressed parts intended for aircraft engines, and the forming of these parts involves forging operations.
2. Summary of the Prior Art
The heat treatments applied to the parts before use normally comprise a solution and hardening treatment, and an annealing treatment. The conditions under which the solution treatment and the subsequent quenching are carried out are known and are not the subject of the present invention. For the sake of example, however, these may involve holding the part at a temperature between 955.degree. C. and 1000.degree. C. for a period of from one to two hours, and quenching with air, oil or water depending on the particular application. The conditions currently observed for carrying out the annealing treatment comprise, depending on the application, either holding the part at 720.degree. C. for eight hours followed by cooling at an hourly rate of 50.degree. C. until a temperature of 620.degree. C. is reached, then holding the part at 620.degree. C. for eight hours before returning it to ambient temperature in conditions equivalent to air cooling, or holding the part at 760.degree. C. for five hours followed by cooling at an hourly rate of 50.degree. C. until a temperature of 650.degree. C. is reached, then holding it at 650.degree. C. for one hour before cooling it with air.
Modifications to this standard treatment have been proposed with a view to obtaining certain particular results. For example, EP-A-O 402 168 proposes a staged heat treatment comprising a tempering treatment followed by a double annealing treatment in conditions close to currently used conditions, without intermediate quenching, so as to avoid the presence of delta phase elements at the grain boundaries. The result obtained is an improvement in the resistance of the alloy to stress corrosion.
Other known processes use thermo-mechanical treatments, some examples of which are disclosed in FR-A-2 089 069 and FR-A-2 099 818. However, the process of FR-A-2 089 069 retains the standard conditions for the annealing treatment of alloy 718, and the process of FR-A-2 099 818 proposes an annealing treatment comprising a six-hour stage at between 700.degree. and 730.degree. C. and a total time of eighteen hours at between 600.degree. C. and 730.degree. C.
The standard conditions for the annealing treatment of alloy 718 described above lead to the restriction of the use of this alloy for parts which are used at temperatures not exceeding 650.degree. C.